There are a number of important systems that require fluid purification, particularly liquid purification. Community water systems, for example, obtain water from local sources, such as lakes and rivers, but such water sources often contain impurities, and also may contain bacteria and other microbiological organisms, that can cause disease. Consequently, water from surface sources must be purified before it can be consumed. Water treatment plants typically clean water by taking it through the following processes: (1) aeration; (2) coagulation; (3) sedimentation; (4) filtration; and (5) disinfection. Portable water purification systems would benefit the production of potable water in areas where there are few if any water treatment plants.
Fluid oxygenators also provide an important example of fluid purification. Oxygenator is the main element of the heart-lung machine, which takes over the work of the lungs by adding oxygen to and removing carbon dioxide from the blood. Inside the oxygenator, blood is channelled along capillary membranes. The inner lumen of the fibres is streamed with oxygen or oxygen enriched air. Oxygen diffuses through the microporous membrane into the blood, while carbon dioxide diffuses out of the blood into the gas stream and is thereby removed. Most oxygenators also include a heat exchanger to maintain the correct temperature of the patient's blood. The oxygenated blood is channelled back to the patient.
Another important example of liquid purification is dialysis. The chemical composition of blood must be controlled to perform its essential functions of bringing nutrients and oxygen to the cells of the body, and carrying waste materials away from those cells. Blood contains particles of many different sizes and types, including cells, proteins, dissolved ions, and organic waste products. Some of these particles, including proteins such as hemoglobin, are essential for the body to function properly. Others, such as urea, a waste product from protein metabolism, must be removed from the blood. Otherwise, they accumulate and interfere with normal metabolic processes. Still other particles, including many of the simple ions dissolved in the blood, are required by the body in certain concentrations that must be tightly regulated, especially when the intake of these chemicals varies.
The kidneys are largely responsible for maintaining the chemistry of the blood by removing harmful particles and regulating the blood's ionic concentrations, while keeping the essential particles. Kidneys act like dialysis units for blood, making use of different particle sizes and specially-maintained concentration gradients. Blood passes through membrane-lined tubules of the kidney, analogous to the dialysis tubes used in dialysis units. Particles that can pass through the membrane pass out of the tubules by diffusion, thus separating the particles that remain in the blood from those that will be removed from the blood and excreted.
Kidneys can effectively maintain the body's chemistry as long as at least ten percent of their functional units are working. Damage to the kidneys that causes the functional capacity to drop below this level may cause fatal illness unless an artificial system performs the work of the kidneys. Without artificial kidney dialysis, toxic wastes build up in the blood and tissues, and cannot be filtered out by the ailing kidneys. This condition is known as uremia, which means literally “urine in the blood.” Tens of thousands of people currently require kidney dialysis, and the number is growing. Kidney dialysis is intrusive, expensive, and complicated. Patients suffer from current treatment protocols due to extensive side effects. Home dialysis is much preferable to the current practice of having patients treated at dialysis centers. Improved technology is needed, however, to make home dialysis feasible and affordable for patients.
Conventional dialysis units are configured as hollow fibers. The membranes are manufactured using spinning technology and generally are about 35μ thick. The membrane is highly porous with the exception of the inner ˜1μ, which actually performs the separation, retaining blood cells but allowing small molecules to diffuse therethrough. These known dialyzers use membranes typically made of cellulose acetate, cuprophan or polysulfone. Blood is pumped through these fibers, and then back into the patient. The membrane has a molecular weight cut-off that allows most solutes in the blood to pass out of the tubing but retains the proteins and cells. Thus, artificial kidney dialysis uses the same chemical principles that are used naturally in the kidneys to maintain the chemical composition of the blood. Diffusion across semipermeable membranes, polarity, and concentration gradients are central to the dialysis process for both natural and artificial kidneys.